In the manufacture of semiconductor devices, in an attempt to provide more devices on a single semiconductor substrate, such as a silicon wafer, several layers of devices are made, separated by dielectric layers. Openings are made in the dielectric layers which are then filled with a conductive material to provide electrical contact between devices in different layers. Such openings are kept small, but they must go through comparatively thick dielectric layers. The ratio of depth to width is called the aspect ratio (AR) and small diameter, deep openings are referred to as high aspect ratio openings. As smaller devices are being made, the AR of such openings increases.
Native oxides are readily formed by exposure of the material at the bottom of the opening to even a very small amount of oxygen; the reaction is spontaneous to form a thin "native oxide" layer. Materials such as silicon, aluminum, copper and the like readily form native oxides from any oxygen that may be present in the chamber used to etch the opening; whether because of exposure of the substrate to air, or to any oxygen-containing species that may be generated during the etch step.
Since oxides are dielectrics, the oxidation of conductive materials such as silicon, aluminum or copper to form their oxides reduces the conductivity of the deposited conductive layer, and creates an interface having a high contact resistance with a subsequently deposited conductive film. This degrades the performance of devices. Thus, in order to prevent degradation of devices, the high aspect ratio openings must be cleaned of these native oxides prior to filling them with a conductive contact material.
Heretofore, native oxides have been removed with a pre-clean step, as by dipping the substrate in a buffered oxide etch, or "BOE" etchant, such as HF and ammonium fluoride, and/or by sputter etching with argon. After removal of the native oxide, however, the substrate cannot be exposed to any oxygen or oxygen-containing gases, or native oxide will re-form.
Thus a method for providing an effective preclean, or removal of native oxide in vacuum that will not re-deposit oxide in the bottom or along the sidewalls of high aspect ratio openings has been sought.
Further, such a method must be able to remove native oxides completely from the bottom of high aspect ratio openings, i.e., having an AR of 4 or higher, without re-depositing native oxide along the sidewalls of the opening.